The present application relates to X-ray imaging systems; and more particularly to such systems having grids to reject scattered radiation.
An apparatus for creating X-ray radiographs is comprised generally of an X-ray source and an X-ray sensitive medium, such as a photographic film and screen combination, for recording an image produced by the varying transmission of X-rays directed through an imaged body. The intensity of a radiographic image at a given point on its surface is ideally a function of the absorptive characteristics of the image body along a straight line from the X-ray tube to that point on the image. For this relationship to hold, X-rays that have not traveled in a straight line from the X-ray tube to the medium, i.e. those that have been scattered within the body, must be blocked to prevent their contribution to the recorded image.
Shielding the medium from scattered X-rays typically is done with a grid that is placed immediately above the medium, such as shown in U.S. Pat. No. 5,040,202. The grid contains channels that are oriented to pass only X-rays proceeding in straight lines from the X-ray tube. These channels are formed by rows of parallel vanes that are constructed of X-ray absorptive material. The vanes are separated either by low obsorptivity solid, such as plastic, or in certain instances by air gaps.
The physical thickness of the grid vanes, as measured along the plane of the X-ray sensitive medium, cause some of the X-rays that would otherwise be passed by the grid to be blocked. The blocking of these X-rays produces shadow "grid lines" in the image. Even fine grid lines may be distracting and larger grid lines can obscure diagnostically significant details in the image. One method of reducing grid lines is to move the grid back and forth parallel to the plane of the X-ray sensitive medium using a DC motor with a cam shaft connected to the grid. The grid shadow thus is blurred by falling on different areas of the medium during the exposure. If the grid can be moved so that each area of the medium is eclipsed by the vane for an equal proportion of the exposure time, the grid lines effectively will be eliminated.
In general, it is quite difficult to move the grid so that its vanes spend an equal amount of time over each area of the mediums. Reciprocating the grid at a constant speed with respect to the medium is one approach. The constant approach speed is upset when the grid changes direction and must be decelerated and then re-accelerated in the opposite direction. In previous reciprocating systems, the grid lines spent a disproportionate amount of dwell time near the ends of their travel, as compared with the center of the travel. As a result, faint grid lines appeared under each vane at the vane's point of direction reversal.
Different techniques have been utilized to reduce the grid line shadows at the end of their reciprocating movement. For example, the aforementioned U.S. Patent describes modulating the X-ray beam synchronously with the grid motion to reduce the grid image at points of grid speed variation. Although this technique was successful, it required additional components of the circuitry for regulating the X-ray beam and a mechanism by which the modulation of the beam was synchronized with the movement of the grid.